Increase of L-thyroxine requirement during pregnancy

In pregnant women with a thyroid disease an increased amount of LT4 may be required for the correction of hypothyroidism or treatment of nodular goiter.

Aim of this study was to assess the amount of the variations of LT4 requirement in pregnant women with thyroid diseases.

To address this issue, we retrospectively evaluated a cohort of 138 women treated with LT4 divided in two groups: 47 euthyroid (E) (affected by nodular goiter (NG) under LT4 suppressive therapy) and 91 hypothyroid (H). This last group was divided in two subgroups: women with a residual functioning thyroid tissue (R-H) and women without residual thyroid tissue (NR-H). In E pregnant women the goal was to maintain TSH serum level between 0.1 and 0.4 mU/L, while in H pregnant women the goal was to maintain the TSH serum level between 0.4 and 4.0 mU/L. 21 E and 48 R-H and 19 NR-H pregnant women respected these criteria during the entire pregnancy.

Only 11 out of 21 (52%) E had to increase LT4 in order to maintain serum TSH in the appropriate range. The mean increase was 125% at 3rd trimester with respect to pre-gravidic dose. In 32 out of 48 (66%) R-H and in 14/19 (74%) NR-H an increase of L-T4 was necessary to maintain serum TSH in the appropriate range. The mean increase was 134% in R-H and 140% in NR-H at 3rd trimester with respect to pre-gravidic dose.

In conclusion, a rise in LT4 dose is required in the minority of pregnant women with NG under suppressive therapy and in the majority of hypothyroid women, especially in those without a residual tissue, in order to maintain TSH serum level in the appropriate range. The increase of LT4 requirement is higher in hypothyroid with respect to NG pregnant women.